Carburizer



patente d June 1, 1943 A zszoszz CARBURIZER Paul H. Kramer and Francis J. Steigerwald, Detroit, Mich, assignors to Par] Chemical Company, Detroit, Mich, a corporation of Michigan No Drawing. Application January 30, 1942, Serial No. 428,922

Claims.

The present invention relates to carburizing materials of the dry pack type.

It has long been common practice to carburize or case harden steel articles by packing them in a granulated carbonaceous material, such as charcoal or coke, in the presence of an activator or catalyst, such as barium carbonate, and raising the mass to a temperature in the order of 1700 F.

In many cases it is necessary to case harden a portion of the area of an article and leave the remaining areas soft, and that result, referred to herein as selective carburization, is customarily accomplished by first copper plating those areas of the article which are not to be hardened and then subjecting the article to the carburizin process. While this method of selective carburizing is successfully practiced, a serious difliculty frequently encountered in its practice is what is known as copper migration. For some hitherto unexplained reason, in certain cases the copper would migrate during carburization from the areas on which it was plated to the areas which were to be carburized. This, naturally, resulted in a hardening of the areas which were required to be soft and a failure to harden the areas which were required to be hardened.

The cause of copper migration has long eluded investigators, partly because of its erratic nature. Thus, with the use of identical materials some commercial carburizers encountered copper migration and others did not. In addition, it was seldom encountered in laboratory practice. Once it was encountered in commercial practice, the only known solution has been to employ a pure or substantially pure charcoal carburizer; but charcoal is relatively expensive and. is subject to excessive shrinkage during the carburizing process. Thus, a charcoal carburizer cannot be reused as often as carburizers made from coke and, in addition, difficulties arise with exposure of the work during carburization due to the excess shrinkage.

Accordingly, it is the general object of the present invention to provide a carburizer which would have all the advantages of low cost, slow burning and high efficiency of the best known coked coal type of carburizer and which will eliminate copper migration during selective carburization.

In the course of the research which led to the present invention, it was discovered that copper migration occurs at relatively low temperatures, i. e. as low as 1100 F., well below the usual temperature of carburization, and that, therefore, it occurs during the initial heating period, rather than at carburizing temperatures. This discovery explained the erratic appearance of copper migration because it indicates that the appearance or degree of migration is a function of the time interval the work is held at these lower temperatures and thus of the rate at which the work is heated to carburizing temperature.

It was further discovered that the phenomenon apparently is due to the presence of sulphur in the coal or coke comprising the carburizing material. Thus, if the heating is interrupted at low temperatures, evidence of copper migration will be noted and the copper will appear as black copper sulphide. Apparently, during normal operations this copper sulphide is reduced to metallic copper at the carburizing temperature, and remains reduced during subsequent cooling.

It might appear that the solution to the problem of copper migration in the light of these discoveries would be to speed up the rate of heating. However, as a practical matter the rate of heating in commercial carburizing plants is already as high as economically possible. Therefore, the only possible solution of the problem lay in the discovery of an addition agent that would in some way fix or neutralize the sulphur in the coal or coke at these lower temperatures without impairing the carburizing process.

In accordance with the present invention, it is found that the sulphur may be fixed or neutralized and copper migration prevented by incorporating calcium carbonate in a coked coal carburizing material which is activated by barium carbonate.

It has long been known that calcium carbonate had some activating properties in the carburizing process, but it is a poor activator and when used alone does not satisfy commercial standards. It is only commercially eifective as an activator when combined with heavy oxides, as disclosed in Rodman Patent No. 2,149,747. If added to commercial activators, such as barium carbonate, it adds little if any to the activation of the carburizing process. Therefore, there has been no occasion for its use, either alone or with other activators, in commercial carburizing materials.

Moreover, it was not to be expected that calcium carbonate would prevent copper migration because when used with heavy oxides as an activator it does not prevent copper migration; nor does it prevent copper migration when coated, along with barium carbonate or any other activator, on soft coal coke.

The carburizing material of the present invention is produced as follows: The calcium carbonate, along with barium carbonate (which serves as the carburizing activator or catalyst), is thoroughly mixed with crushed soft coking coal, and the resulting mixture coked and then recrushed and screened to the desired sizes. This results in a carbonaceous material of high carburizirrg ability which is low in cost and long burning. It is essential that the calcium carbonate be mixed with the coal before the coal is coked, as a mere coating of the finished coke will not produce the desired result. This may be due to the fact that when calcium carbonate is mixed with the coal before it is coked the carbonate is dispersed through each coke particle in more intimate relation to the sulphur than would be possible with a mere coating. In certain of the claims it is stated that the coke is impregnated with calcium carbonate or both calcium and barium carbonates. By this expression is meant that dispersion of these materials through each coke particle which results from mixing the materials prior to coking, as distinguished from a coating of the coke With the materials.

To obtain satisfactory results, it is advisable to employ a high volatile coal having a low ash and sulphur content. 4

Conventional amounts of barium carbonate are preferably employed in the material. That is to say, the slight activating effect of the calcium carbonate is preferably ignored and the same quantity of barium carbonate used as has been used in prior barium carbonate impregnated coke carburizers. The activity of barium carbonate in such a carburizer increases with the barium carbonate content until about thirty per cent barium carbonate is reached, but maximum activity is seldom, if ever, desired because it produces too high a concentration of carbon in the work. The preferred quantity of barium carbonate for normal activity is approximately l4%-1'7% in the coked material, but greater or lesser amounts may be used if greater or lesser activity is desired. As little as 7% will effectively activate the carburizer, particularly when calcium carbonate is present in accordance with this invention.

It should be noted that the percentages given herein are those of the ingredients in the cornpleted carburizer. During the coking operation approximately one-third or more of the coal is lost as volatile matter and, therefore, proper allowance for this loss must be made in determining the quantity of chemicals to be mixed with the coal prior to coking.

It is to be understood that in place of barium carbonate, anhydrous barium oxide or hydrated barium oxide (barium hydroxide) in corresponding quantities may be used as an activator. In addition, corresponding quantities of calcium oxide, either in its anhydrous or its hydrated form, may be employed in place of calcium carbonate. However, it has been noted that when calcium oxide, or its hydrated form calcium hydroxide is employed in lieu of calcium carbonate, for some unknown reason copper migra tion may occur during the first heat of the carburizing material. Thereafter, no copper migration occurs,

t should be mentioned that it is not known in what form the above mentioned barium and calcium compounds appear in the carburizing material after the coking operation. They may be carbonates, oxides, sulphates, sulphides or silicates or mixtures of two or more such comiii is present.

pounds. The uncertainty is due to the fact that it is impossible to dissolve out all of the barium and calcium compounds with either Water or acid when the material is in its coked condition. Accordingly, the customary method of analysis involves first ashing the coked material. However, during ashing the nature of the compounds is subject to change, the amount of change depending upon the temperature at which the material is ashed. In particular, some or all of any calcium carbonate present becomes cal- Cium oxide. Accordingly, it is to be understood that when, in the appended claims, it is stated that the material contains barium and calcium carbonates or oxides, it is meant that it contains such barium and calcium compounds as inherently result from a coking of the carbonates or oxides with the coal.

The quantity of calcium carbonate employed may be varied, depending on the conditions. It is found that about 15% in the finished carburizer will positively prevent copper migration under all conditions if the best quality coking coal is used. Additional amounts up to about 25% may be used-ii the coal has a higher sulphur content. A greater amount than necessary can be used since calcium carbonate is cheap and may be used as a diluent. Amounts in excess of about 25% should be avoided because they weaken the coke particles. Lesser amounts may be used successfully, depending upon the amount of sulphur in the coal and the rate at which the work is brought up to carburizing temperature. If the best quality coking coal is, used, approximately '7 calcium carbonate will prevent copper migration under all ordinary conditions, although with that quantity copper migration will still occur if the work is heated to carburizing temperature at an extremely slow rate of speed.

If, as is possible under laboratory conditions and some commercial conditions, the work is heated through the copper migration temperature range very rapidly, no appreciable copper migration will occur even though no calcium carbonate It is only because commercial considerations often prevent such rapid heating that the problem of copper migration exists; and the difficulty is one of degree, depending upon the rate of heating employed. Therefore, the amount of calcium carbonate required may theoretically reduce alomts to zero in particular cases. However, as a practical matter, since excess quantities are harmless it is preferred to employ from 7%-15% calcium carbonate in the finished product.

It will be understood that the only essential ingredient other than the above mentioned barium and calcium compounds is a coking coal, but, of course, other materials may be included without departing from the spirit of the invention.

What is claimed is:

l. A carburizing material characterized by its property of preventing copper migration during selective carburizing, comprising soft coal coke impregnated with between 7% and 25% of a material from the group consisting of calcium carbonate and calcium oxide and combined with a carburizing activator from a group consisting of barium carbonate and barium oxide.

2. A carburizing material characterized by its property of preventing copper migration during selective carburizing, comprising a coked mixture of coking coal with between 7% and 25% of a material from a group consisting of calcium carbonate and calcium oxide and an effective amount the group consisting of barium carbonate and barium oxide.

4. The method of carburizing a limited area of a steel article which includes coating those areas 1 of the articles which are not to be carburized with a layer of copper plate, embedding the coated article in a mass of material comprising a coked mixture of coking coal with from 7% to 25% of a material from a group consisting of calcium carbonate and calcium oxide and from 7% to 30% of a carburizing activator from the group consisting of barium carbonate and barium oxide, and raising the material and embedded articles to a temperature in the orderof 1700 F.

5. The method of making a carburizing material characterized by its property of preventing copper migration during selective carburizing which consists in mixing crushed coal with from 4% to 15% of a material from a group consisting of calcium carbonate and calcium oxide and from 4% to 18% of a carburizing activator from the group consisting of barium carbonate and barium oxide, and coking the resultant mixture.

PAUL H. KRAMER. FRANCIS J. STEIGERWALD. 

